Ask A Scientist Chemistry Archive

     
name         Michael K.
status       educator
age          40s

Question -   Does carbon monoxide rise or sink if produced inside a
house?  Why?  Does natural gas rise or sink?
(I am trying to figure out where to position gas detectors).
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Although the molecular weights of these gases differ, convection and not
density differences dominate the distribution of CO, methane etc. The
release of gases associated with combustion tend to rise because they are
hotter than ambient. For this reason smoke detectors and CO detectors are
usually placed high on the walls of hallways etc.

Vince Calder
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Michael,

High or low in a room; It makes no difference where you put the detector 
-- but not because carbon monoxide and air have approximately the same 
density. Both are gases and as a result, diffusion alone and the entropy 
effect will take care of mixing them. Any convection currents and/or 
forced-air turbulence present will also enable the gases to mix and affect 
the detector.

It is popularly misconceived that light (low-density) gases will somehow 
float atop heavy (higher density) gases. Indeed, if the higher density gas 
was admitted low in the room, and done so in a manner that would not cause 
much mixing, it would take a while for the gases to become thoroughly 
mixed by the mechanisms mentioned above. Even so, they would eventually mix.

That said, since time to detection may be of the essence, and in 
circumstances where the contaminant gas (such as radon -- a gas of very 
high density) gets into the room through entry points more or less at 
floor level, where one puts the detector can become important.

Regards,
ProfHoff 909
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Air is 1/4 oxygen O2, (weight 16) and mainly nitrogen N2 (weight 14).

CO is weight 12+8 = 20, so heavier than "air"
Methane (natural gas) is CH4 = 12 + 4(1) = 16, so mixes with air fairly
well, hard to say.

I would put the CO and methane detectors near the furnace.

Steve Ross
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Neither, for the most part.
Carbon monoxide has nearly the same density as air.
You'll notice that the manufacturer's instructions that come with the 
detector don't say high or low either.

The density of a gas is proportional to the weight of a single molecule of 
that gas.

So you figure relative buoyancy for yourself if you start learning a few 
atomic weights:
H=1,  He=4,   C=12,  N=14,  O=16.

Add them up for the molecular weights of pure gasses:
     H2 = (1+1)     =  2,  very light
     He = (4)       =  4,  very light
     N2 = (14+14 )  = 28,  about neutral
     O2 = (16+16)   = 32,  slightly heavy
     CO2= (12+16+16)= 44,  heavy
     CO = (12+16)   = 28,  about neutral
     CH4= (12+4*1)  = 18,  light   (majority part of natural gas)
     H2O= (2*1+16)  = 18,  light   (steam)
     C2H6=(2*12+6*1)= 30,  about neutral (minority part of natural gas)
     C3H8=(8 + 3*12)= 42,  heavy  (propane)
     C4H10=(10+4*12)= 58,  (butane)
     C5H12=(12+5*12)= 70,  pentane, lightest part of gasoline

For mixed gasses just take a proportionate average:
Air is 80% N2 + 20% O2 .
     air  = 0.8(28) + 0.2(32) = 29  (exactly neutral, by definition)

So pure carbon monoxide is actually about 3% lighter than air.
But usually it is made in modest concentrations, mixed in with the normal 
combustion products:  CO2, H2O.
Which are always mixed with the 80% Nitrogen that never participates in 
burning.
Then that mixes with room air, making an even smaller concentration...

And there are uncertainties...
Some fuels make light exhaust (more H2O), some make heavy (more CO2).
Then when the exhaust cools the light part, H20 (steam), may condense and 
drop out.
Not to mention that the exhaust gas was expanded when hot, and it 
contracts as it cools.
No rule can predict which way it is going to go in most circumstances.

Because it travels in whatever directions your air normally 
circulates,  which varies,
it is difficult in most homes to find an advantageous position where CO 
will enter or concentrate.
So we settle for any convenient position, or one associated with that 
which we wish to protect,
namely us, breathing in air at medium heights.

My CO detector is at about waist-level in a hallway, partly because there 
was a convenient power socket there.
Also the only air-intake to the house heater is there, so virtually all 
the air in the house will pass that position regularly.

Doing a really good job needs more than one detector, but they are not 
cheap enough for that yet.
One for each potential source of monoxide, plus one for each 
protectee.  Maybe someday.
Meanwhile, if I had a wall heater, I wouldd definitely place my one 
detector near there, probably high up.

Jim Swenson

PS- natural gas rises.
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Up-date 2/4/2005
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Just read your reply to Michael K. on "Carbon Monoxide and Density" The 
furnace room location may be okay if you have more than one alarm. However 
of more prime concern is that the occupants be able to hear the alarm 
while asleep. National Fire Protection Association has a code , number 
720, 2-1.1.2* 1998 states that " A carbon monoxide alarm or detector 
should be centrally located outside of each separated sleeping area in the 
immediate vicinity of the bedrooms. Where bedrooms are separated and the 
audibility of the alarm or detector to occupants within the bedroom area 
could be seriously impaired, more than one unit could be needed. Each 
alarm or detector should be located on the wall, ceiling, or other 
location as specified in the installation instructions that accompany the 
unit." . In addition more than CO will be emitted with furnace failure or 
with the use of an unvented heaters. Carbon dioxide a heavier gas will 
collect on the floor building up and possibly preventing carbon monoxide 
from ever reaching the alarm if located at a floor outlet. That is why it 
is recommended that CO alarms be placed high around eye level where the 
higher concentrations would be concentrated. You are correct that natural 
gas rises, however Michael K. did not state if his question on the gas 
detector if it was  about natural gas or propane gas. If his residence is 
using propane then he would need to place the gas detector on the floor 
next to the furnace and gas water heater.  It is my hope that you ammend 
your answers to Michael K.

Gary R.
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